1209 Remove All Adjacent Duplicates in String II

You are given a string s and an integer k, a k duplicate removal consists of choosing k adjacent and equal letters from s and removing them, causing the left and the right side of the deleted substring to concatenate together.

We repeatedly make k duplicate removals on s until we no longer can.

Return the final string after all such duplicate removals have been made. It is guaranteed that the answer is unique.

Example 1:

Input: s = "abcd", k = 2
Output: "abcd"
Explanation: There's nothing to delete.

Example 2:

Input: s = "deeedbbcccbdaa", k = 3
Output: "aa"
Explanation: 
First delete "eee" and "ccc", get "ddbbbdaa"
Then delete "bbb", get "dddaa"
Finally delete "ddd", get "aa"

Example 3:

Input: s = "pbbcggttciiippooaais", k = 2
Output: "ps"

Constraints:

• 1 <= s.length <= 105
• 2 <= k <= 104
• s only contains lowercase English letters.

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class Solution:
    def removeDuplicates(self, s: str, k: int) -> str:
        stack = []  # [count, char]
        for c in s:
            if not stack or c != stack[-1][1]:
                stack.append([1, c])
            else:
                stack[-1][0] += 1
                if stack[-1][0] == k:
                    stack.pop()
        return ''.join([char * count for count, char in stack])